of Radiation by Electron Impacts in Helium. 557 



one end of a hollow copper cylinder, shaped like a napkin- 

 ring, with platinum foil /and covering the other end with 

 platinum gauze g. The cylinder is suspended by a platinum 

 wire ending in a swivel, so that it can be rotated about its 



Fig. 1. 



vertical axis by the aid of a horseshoe magnet acting on a 

 soft- iron block I, which is rigidly attached to the cylinder by 

 a light brass rod. Evidently it is a matter of indifference, 

 as regards the receiving of positive ions, whether the foil 

 end /or the gauze end g of the electrode E is turned to face 

 the filament. But the two ends are differently affected by 

 ultra-violet radiation ; for, when the gauze side is exposed, a 

 large part of. the radiation passes through into the cylinder, 

 and does not result in the loss of electrons from the cylinder. 

 Thus the ratio R = E//E^ of the electrometer deflexion with 

 the foil end exposed, and the deflexion with the gauze end 

 exposed, determines the proportion of the total observed 

 effect which is due to true ionization or to ultra-violet 

 radiation. 



The calculation of this proportion is made very easily as 

 follows: — Let i and r be the rates at which the electrode E 

 grains a charge due to ionization and to photoelectric effect 



